The UsersOnline Macro displayes the following:

Users Online:
 Members: 5
 Visitors: 7
 Total: 12

 Members Online:
<Link to online Users' profiles>

If no registred users are online or none of the online users allow their profiles to be visible here the members online will not be displayed.

This visibility can be edited in the user profile:
Show when online? yes/no

Visitors cannot see which members are online.